1. Field of the Invention
The present invention relates to optical communication, and in particular, to optical network restoration.
2. Related Art
A communication network serves to transport information among a number of locations. The information to be transported is often presented to the network in the form of time-domain electrical signals representing a combination of telephony, video, or computer data in a variety of formats. A typical communication network has many physical sites called nodes interconnected by information conduits called paths, also called spans. Each path or span carries data from one node to another node. A path or span can include one or more links. Each node can contain equipment for combining, separating, transforming, conditioning, and routing data.
Optical networks having optical fiber links are increasingly relied upon for carrying vital communications traffic at a high volume. Fiber optic cables carry far greater amounts of digital data than conventional electrical cables. A single fiber operating at approximately 10 Gigabits/sec (Gb/s) and transmitting data according to a high-speed synchronous digital hierarchy standard, such as the SONET OC-192 protocol, carries data equivalent to 129,024 voice calls and up to 16 OC-192 channels on a single fiber pair with wavelength division multiplexing. Moreover, dozens of fibers may be included in a single cable. The impact of a cable cut, or even a single optical fiber failure, can be widespread. Sudden link failure due to a fiber failure, cable cut, nodal failure, or any other system failure can cause a significant loss in revenue for a network owner or network subscriber. Sophisticated consumers no longer tolerate disruptions of service. Prompt restoration of optical signal impairment or loss is therefore essential to effective network management.
Optical networks can include endpoint nodes optically interconnected in one or more point-to-point links or through paths which include intermediate optical switching nodes. Optical switching nodes are also called optical cross-connect switches. Each path can include multiple intermediate nodes coupled to one another through multiple spans or links in a variety of topologies, such as, point-to-point, star, ring, mesh, spoke-hub, or any combination or variation thereof.
To maintain service integrity, optical restoration paths are provided in a physically separated route between optical switching nodes in the optical domain of an optical communication network. Working traffic traveling along a span between two optical switching nodes is switched to an optical restoration path in the event restoration is needed. For example, restoration may be triggered by a link failure, fiber degradation or cut, or any other massive equipment failure along a working path between the optical switching nodes. After a failure is repaired, traffic is switched from a restoration path back to a working path in a process known as "normalization."
Optical amplifiers and light regenerators are generally provided along working optical paths, such as long-haul fiber links, so that optical signals can be carried over greater distances. For the same reason it is desirable that optical restoration paths also include optical amplifiers and/or light regenerators. Optical restoration paths, however, are typically idle with no traffic signals present. Light regenerators placed in an optical restoration path would issue false alarms during idle periods when restoration is not needed and light is not received from line terminating equipment. Delay would be experienced if optical amplifiers are placed in an optical restoration path as the amplified output of one or more optical amplifiers would have to ramp up to an operating condition when traffic is switched to the restoration path. The output of a single optical amplifier can take 10 to 30 milliseconds to ramp-up after an input signal is received. The overall ramp-up delay effect is even greater for a series of line optical amplifiers. Delay is also experienced in the normalization of optical amplifiers in a working path as the optical amplifiers may not have received any input light signals during a restoration event.
What is needed is a method and system that squelches false alarms by components in an optical restoration path which are sensitive to loss of light such as light regenerators. Further, a method and system is needed that stabilizes components such as optical amplifiers in an optical restoration path even when the optical restoration path is idle. Similarly, a method and system is needed that squelches false alarms and stabilizes components in an optical working path prior to normalization.